This invention relates to electrical conductors based on organic structures and more particularly to organic superconductors with improved transition temperatures at ambient pressure.
Electrical conductors based on organic structures and particularly organic superconductors represent a relatively new area of development compared to the metallic counterparts. While some metallic superconductors such as niobium and alloys of niobium have been used commercially for fabricating coils for supermagnets, organic superconductors are in a relatively early stage of development. Organic superconductors have been in general referred to as organic metals or synthetic metals because they have metal-like electrical conductivity which does not derive from the electrons of metal atoms. They also may be varied in structure by changes in composition and in general have a lower density than the metallic superconductors.
The first organic superconductor was (TMTSF).sub.2 X where TMTSF is the selenium-based organic donor tetramethyltetraselenafulvalene and X is a complex univalent anion such as ClO.sub.4.sup.- or PF.sub.6.sup.-. The superconducting properties of these salts were first observed in 1980. These salts are now identified as Bechgaard salts.
In general, the Bechgaard salts undergo metal-to-insulator transitions as temperatures are decreased. For some of the Bechgaard salts, these transitions may be suppressed by applied pressures of several thousands of atmospheres to achieve a superconducting state near 1 K. In 1981, Bechgaard and coworkers reported that (TMTSF).sub.2 ClO.sub.4 exhibits ambient-pressure superconductivity with a transition temperature of about 1.2 K.
A more recent cation donor which produces organic superconductors is BEDT-TTF or ET which represents bis(ethylenedithio)tetrathiafulvalene. With ClO.sub.4.sup.- as the anion in (ET).sub.2 ClO.sub.4 (C.sub.2 H.sub.3 Cl.sub.3).sub.0.5, the material reportedly has metallic conductivity but is not superconducting down to T=1.4 K. (ET).sub.2 ReO.sub.4 is reportedly superconducting at 4000 atmospheres with a transition temperature of about 2 K. When the anion (I.sub.3.sup.-) was substituted by Russian researchers to produce .beta.-(ET).sub.2 I.sub.3, superconductivity at ambient pressure was achieved near 1.4 K. Subsequently, .beta.-(ET).sub.2 IBr.sub.2 was prepared which exhibits a transition temperature of 2.7 K.
One difference between these compositions was the reduction by about 7% in the total length of the anion (trihalide). This difference was expected to decrease the size of the unit cell in .beta.-(ET).sub.2 IBr.sub.2 and increase the sulfur atom orbital overlap. The orbital overlap in the sulfur atom network constitutes the conductive band in these ET-based organic metals. Arguments based on increased overlap would predict a lower transition temperature for .beta.-(ET).sub.2 IBr.sub.2. When a higher temperature was observed, the results were somewhat surprising. Subsequently, in the hope of achieving a higher T.sub.c, a smaller anion derivative, (ET).sub.2 ICl.sub.2, was synthesized but exhibited no superconducting properties.
While the above work has provided significant results, it is of particular importance to increase transition temperatures to at least above the boiling temperature of helium (4.2 K). Helium could then be used to control the operating temperature of the organic superconductor.
Accordingly, one object of the invention is new organic superconductors. A second obJect of the invention is organic superconductors with increased transition temperatures. Another object of the invention is organic superconductors with a transition temperature above 4.2 K. These and other objects will become apparent from the following description.